Synthetic colour hologram

ABSTRACT

A surface relief hologram that comprises two or more patterns, each pattern being sensitive to different radiation wavelengths. The hologram is made by defining relief features that are sensitive to different wavelengths, and in particular to one wavelength that is visible to the unaided human eye and to one wavelength that is invisible to the unaided human eye. These relief features of different sensitivities are interspersed over the surface of the substrate in which the hologram is defined. By providing two patterns of different sensitivities, when the hologram is used in a security device, security is improved. Despite this improved security, the hologram can be mass manufactured using known techniques.

FIELD OF THE INVENTION

The present invention relates to a hologram and a method for making sucha hologram. In particular, the invention relates to a hologram for usein a security device.

BACKGROUND

Using holograms to provide some level of document security is wellknown. For example, many bankcards carry a holographic image of theauthentic card user, so that the identity of that user can be verified.In other cases, holograms are embedded within security documents, sothat they are invisible to the unaided eye. To verify or authenticatesuch documents, the hologram has to be irradiated with light of asuitable wavelength. Depending on the wavelength used, the holographicimage can either be viewed directly or it can be sensed and imaged usingsuitable imaging techniques.

In the drive towards increased security, many hologram based validationor authentication techniques are becoming more complex. However, aproblem with increasing the complexity of security is that it almostinvariably increases the complexity of the procedures required tomanufacture the security device and/or the hologram itself. This limitsthe use of such systems, particularly for applications where massmanufacture techniques are needed in order to make the devicescommercially viable.

SUMMARY

An object of the present invention is to provide a hologram thatovercomes at least some of the problems associated with known holograms.

Another object of the invention is to provide a hologram based securitydevice that can be mass manufactured.

According to one aspect of the invention, there is provided a surfacerelief hologram that defines two or more patterns, each pattern beingsensitive to radiation of different wavelengths one pattern beingsensitive to visible radiation and the other pattern being sensitive toinvisible radiation. A hologram of this nature can be made usingstandard processing techniques, and so is capable of mass manufacture.In addition, for security applications, by providing two or morepatterns that are sensitive to different wavelengths, an additionalbarrier to copying is provided, so that security is enhanced.

Each pattern is defined by relief features that are formed in a suitablesubstrate. The relief features for each pattern are of differentdimensions, so that one pattern is sensitive to light of a firstwavelength and the other pattern is sensitive to light at anotherwavelength. Preferably, the relief features of both patterns areinterspersed over the surface of the substrate. Optionally, more thantwo patterns may be provided, each being sensitive to radiation ofdifferent wavelengths.

Preferably the surface relief features that are sensitive to invisibleradiation are sensitive to UV or IR radiation.

According to another aspect of the invention, there is provided asecurity device or label that includes a surface relief hologram thatdefines two or more patterns, each pattern being sensitive to radiationof different wavelengths, preferably wherein one pattern is sensitive tovisible radiation and the other pattern is sensitive to invisibleradiation.

According to yet another aspect of the invention there is provided amethod for making a surface relief hologram comprising defining aplurality of surface relief features that have dimensions that aresensitive to radiation of different wavelengths. By defining features ofdiffering sensitivities, a dual colour hologram can be made in a verysimple and effective manner.

Preferably the surface relief features are defined using standardmanufacturing techniques including lithography, such asphoto-lithography or e-beam lithography, and etching, for example wetetching or dry etching, in particular reactive ion etching.

Preferably, the method further involves selecting two or morewavelengths of interest and using these to determine the dimensions ofthe surface relief features. The wavelengths of interest may be visibleand/or UV and/or IR radiation. Preferably, the wavelengths of interestinclude one or more wavelength that is visible to the unaided eye andone or more wavelength that is invisible to the unaided eye.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention will now be described by way of exampleonly and with reference to the accompanying drawings, of which:

FIG. 1 is a schematic representation of a process for making a dualcolour surface relief hologram, and

FIG. 2 is schematic representation of the use of a hologram made inaccordance with the methodology of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows the steps that are taken to make a surface relief hologramin which the invention is embodied. This is based on well-knowntechniques that use VLSI technology. The method involves defining aseries of masks and using these to form relief patterns on a suitablesubstrate. As is well known in this field, the number of masks requireddepends on the number of levels of the structure. For a sixteen levelstructure, four masks are needed, whereas for a structure having twohundred and fifty six levels, eight different masks are needed. Thefinal hologram is more light efficient if the number of masks used isrelatively high. However, structures having a high number of levels aredifficult to fabricate. Hence, there will always be some form ofcompromise between ease of fabrication and light efficiency. For thesake of simplicity, in the example of FIG. 1, an eight level structureis shown, which structure is defined using three separate masks.

As a first step, the wavelengths of operation are selected, for exampleblue and red. Ideally, the wavelengths selected should be spectrallywell separated and easily distinguishable visually. Then, the masks thatare used to define the hologram are generated or devised using anysuitable technique, but preferably using computer software that performsan iterative Fourier transform algorithm for design optimisation.Software of this nature is well known. In this case, however, thealgorithm is adapted to do a two-stage optimisation to take into accountthe relative phase information for the two selected wavelengths ofoperation. More specifically, as part of the mask optimisation process,the depth of the surface relief features is defined, so that some of thefeatures are sensitive to a first wavelength and some are sensitive to asecond wavelength.

The depth or height of the surface relief features and the refractiveindex of the material in which they are formed define the wavelengths towhich they are sensitive. By defining different depths of differentfeatures, some features can be made sensitive to radiation of a firstwavelength and others can be made sensitive to radiation of a secondwavelength. As is well known, the step size for a surface reliefhologram has to be a multiple or function of the wavelength of theilluminating light. Hence, to define patterns having differentsensitivities, different etch depths, or more specifically differentstep heights, are required. Typically, the overall surface relief depthof the features defined is around 2 to 5 microns.

Following the mask optimisation stage, data files representing thetwo-dimensional masks are created. These are used to form the masksusing any suitable technique, such as photolithography or e-beamlithography. Once the masks are designed, they can be used to form thesurface relief hologram. This can be done using either photolithographyor e-beam techniques. However, for the sake of clarity, onlyphotolithography techniques will be described.

FIG. 1 shows various stages in a process for defining a surface reliefhologram. Firstly, a first one of the masks is formed, and resist isapplied to a suitable substrate, for example silica or glass or metal,see FIG. 1(a). The first mask is applied over the resist and thesubstrate is illuminated using suitable radiation, typically UVradiation. Then, the resist is developed and the surface is etched usingany suitable technique, such as wet or dry etching, in particularreactive ion etching, to remove exposed material, see FIG. 1(b). In thisway, a first step is defined, and so two different levels. The firstmask is then removed, as well as the remaining resist, as shown in FIG.1(c).

After the first mask is removed, a second layer of resist is applied.The second mask is then placed over the resist-covered substrate asshown in FIG. 1(d). As before areas of the resist that are not coveredby the mask are exposed and developed, and the surface is then etched.The second mask is such that after etching, three steps are defined inthe substrate, as can be seen in FIGS. 1(e) and (f), thereby definingfour different levels. This process is then repeated with the thirdmask, so that four more levels are defined, thereby resulting in astructure having eight levels, see FIGS. 1(g) to (i).

In order to define features having different wavelength sensitivities,the etch depths for the different etch stages are varied. As notedabove, the depth of the features needed to ensure sensitivity atdifferent wavelengths is determined as part of the optimisation process.To ensure that the required step sizes are defined, in practice, thestep heights are measured at each stage using a profilometer to ensureclose proximity to the theoretical values calculated using theoptimisation process.

Because the hologram defined using the steps illustrated in FIG. 1 hasfeatures that are of different dimensions, when the hologram isilluminated with radiation of suitable wavelengths, different patternscan be viewed. For example, in the case where blue light and red lightare chosen as the wavelengths of interest, when the hologram isilluminated with red light, only the red pattern can be seen. This isshown as Pattern 1 in FIG. 2(a). Likewise, when the hologram isilluminated with blue radiation, only the blue pattern can be seen, seePattern 2 in FIG. 2(b). Of course, when the hologram is simultaneouslyilluminated with red and blue radiation, both patterns can be seen, seeFIG. 2(c).

The main property of the hologram in which the invention is embodied isthat it is a single diffractive surface relief element that canreconstruct distinct intensity patterns in the far field on illuminationwith light of different wavelengths. As will be appreciated, it could bedesigned to work in either a reflection or a transmission mode,depending on what is most convenient for a particular application. It isalso capable of mass manufacture using a variety of replication andembossing techniques, so that many copies of the hologram can be made ina relatively simple manner. Techniques for replicating surface reliefholograms are well known. In addition, assuming the wavelengths havebeen appropriately selected, the encoded patterns can be interrogatedusing very low cost and low power light emitting diodes. All of thesefeatures widen the scope for practical applications.

The surface relief hologram described above can be used in a securitylabel or device. The hologram can be incorporated into the securitydevice in any suitable manner. For example, the hologram may be providedin a laminated structure, such as a credit card or other security card.Of course, any coating applied to the hologram would have to betransmissive to the wavelengths that the hologram is sensitive to.Because it is relatively easy to replicate from a manufacturingperspective, but difficult to interpret the patterns without knowledgeof both of the wavelengths, there is provided a high level of securityin a device that can be readily mass manufactured.

A skilled person will appreciate that variations of the disclosedarrangements are possible without departing from the invention. Forexample, whilst the specific embodiment relates to a hologram in whichboth patterns are sensitive to visible light, the patterns could equallyboth be sensitive to light that is invisible to the unaided eye.Alternatively, one pattern may be sensitive to radiation that is visibleto the unaided eye and the other may be sensitive to radiation that isinvisible to the unaided eye. The invisible radiation may be UVradiation or IR radiation. Of course, in either case to detect thepattern that is sensitive to the invisible radiation, suitable detectorand imaging equipment is needed so that the pattern can be detected, andultimately viewed by an authorized party. Accordingly the abovedescription of a specific embodiment is made by way of example only andnot for the purposes of limitation. It will be clear to the skilledperson that minor modifications may be made without significant changesto the operation described.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiments but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures as is permitted under the law.

1. A hologram that defines at least two patterns, each pattern beingsensitive to radiation of different wavelengths, wherein one pattern issensitive to radiation that is visible to the unaided eye and the otheris sensitive to radiation that is invisible to the unaided eye.
 2. Ahologram as claimed in claim 1 wherein the hologram is a surface reliefhologram, and each pattern is defined by surface relief features thatare of different dimensions.
 3. A hologram as claimed in claim 1 whereinthe pattern that is sensitive to invisible radiation is sensitive to UVradiation or IR radiation.
 4. (canceled)
 5. A method for making asurface relief hologram comprising defining a plurality of surfacerelief features, some of the surface relief features being sensitive tovisible radiation and others being sensitive to invisible radiation. 6.A method as claimed in claim 5, wherein the surface relief features aredefined using standard manufacturing techniques including lithography,such as photo-lithography or e-beam lithography, and etching, forexample wet etching or dry etching, in particular reactive ion etching.7. A copy of a surface relief hologram that is made according to themethod defined in claim
 5. 8. A security device or label that includes asurface relief hologram, that defines a plurality of patterns, eachpattern being sensitive to radiation of different wavelengths.
 9. Asecurity device or label as claimed in claim 8, wherein one pattern issensitive to radiation that is visible to the unaided eye and the otheris sensitive to radiation that is invisible to the unaided eye.
 10. Asecurity device as claimed in claim 9, wherein the pattern that issensitive to invisible radiation is sensitive to UV radiation or IRradiation.
 11. (canceled)